1. Field
Example embodiments relate to phase-change random access memories (PRAMs), for example, PRAMs having a relatively low crystallization temperature, and relatively quick operation speed. Example embodiments also relate to memory systems capable of performing a refresh operation and semiconductor memory devices capable of suppressing data loss. Example embodiments also relate to methods of operating and methods of manufacturing semiconductor memory devices
2. Description of the Related Art
A phase-change random access memory (PRAM) uses characteristics of a phase-change material, such as GeSbTe, to store data. For example, when an electrical pulse is applied to the phase-change material, a state of the phase-change material changes between crystalline and amorphous states according to heat generated in a localized area. A memory cell, which memorizes PRAM binary information, includes a phase-change layer, a bottom electrode contact (BEC) layer, and a switching device, such as a transistor.
In a conventional PRAM, a transistor is formed on a silicon wafer, and a BEC layer is connected to a source or a drain of the transistor. A conventional phase change layer is formed of a GST (GeSbTe) based material such as a Ge2Sb2Te5 composition. The GST based material is referred to as a chalcogenide, which is the same material used in an optical recording apparatus such as a digital video disc (DVD) or a re-writable compact disc (CD-RW). The BEC layer is formed in order to heat the phase-change layer. According to the extent of the heating of the phase-change layer, the phase of the conventional PRAM is changed between crystalline and amorphous states, and a resistance value changes according to the state of the conventional PRAM. Binary information may be stored and read because a current or voltage changes according to resistance of the phase-change layer.
Operating speeds of PRAMs depend on the speed at which the phase-change layer transitions between a crystalline structure and an amorphous structure. A GST material used in conventional PRAMs has a relatively high crystallization temperature, and relatively slow crystallization speed. Accordingly, when the PRAM changes from a set state having a crystalline structure to a reset state having an amorphous structure, the required reset current is relatively high. Also, when the PRAM changes from a reset state having an amorphous structure to a set state having a crystalline structure, the required set time is relatively long (e.g., greater than or equal to about 100 ns). Accordingly, it is relatively difficult to realize a PRAM that operates at relatively high speeds.